The present invention relates to a liquid crystal display device driving method for improving the display quality of dynamic images (moving images).
In recent years, the liquid crystal display that employs a matrix type liquid crystal display device has a spreading market in a variety of commercial fields as a display device for a television set inclusive of OA (Office Automation) equipment taking advantage of its features of a thin configuration, light weight and low consumption of power. According to this trend, the liquid crystal display is used for displaying not only characters and pictures but also dynamic images such as images based on a television signal and a video signal. However, in the present circumstances, the liquid crystal display cannot obtain vivid images in displaying dynamic images in comparison with the CRT (Cathode Ray Tube) type display. The liquid crystals employed in the liquid crystal display have a slower response speed with regard to its transmittance to the applied voltage and a poor charge characteristic in compliance with the change in permittivity of the liquid crystals and are accordingly unable to sufficiently respond to rapid changes in the image signal.
In order to improve the aforementioned drawbacks with regard to the dynamic image display, National Publication of the Translation No. No. HEI 8-500915 turns on the backlight illumination for displaying the image written in the liquid crystal display device only in a part of time for display and is provided with a dark period with the backlight illumination turned off in the remaining part of time. By so doing, the image is visually perceived as if it moved smoothly, improving the dynamic image display.
The transmittance of liquid crystals changes as a consequence of the change in the orientation of liquid crystal molecules due to the written (applied) voltage. However, the permittivity also changes when the orientation of the liquid crystal molecules change, and the value of the applied voltage accordingly changes due to the change in the permittivity. Therefore, in order to obtain a specified transmittance, it is required to repetitively supply the voltage during several vertical synchronization intervals, and the liquid crystals are to have a step response characteristic. As a method for improving the reduction in the response speed of liquid crystals due to this step response characteristic, Japanese Patent Laid-Open Publication No. HEI 6-62355 discloses the improvement in the step response characteristic of liquid crystals by superimposing a difference component by comparison with the previous image signal.
However, the aforementioned conventional method for improving the drawback of dynamic image display has the problems as follows. That is, in the case of National Publication of the Translation No. HEI 8-500915 in which the backlight illumination is turned on only in a part of time, there is a problem that the image becomes dark as a consequence of the reduction in illuminance of the liquid crystal display device due to the occurrence of a period during which the backlight is turned off. Moreover, there is another problem that the image signal of the previous frame is visually superimposed since the response speed of the liquid crystals is not improved, resulting in a double or triple vision.
In the case of Japanese Patent Laid-Open Publication No. HEI 6-62355 in which the component of difference with respect to the previous image signal is superimposed in repetitively supplying the voltage during several vertical synchronization intervals, the response characteristic of the liquid crystals is utterly insufficient for display within one vertical synchronization interval. Even if the illumination is darkened during a part of the period as in the case of, for example, Japanese Patent Publication No. HEI 8-500915, there is a problem that the period during which the change in the liquid crystals is insufficient is disadvantageously displayed. Moreover, it is required to increase the value of the voltage to be superimposed in order to make the liquid crystals have a rapid response, in this case the transmittance becomes larger than the intended transmittance. Accordingly, there arises the need for restoring the transmittance in the next one vertical synchronization interval, and this consequently leads to a reverse step response, causing a problem that the response characteristic is not improved.